Tuesday, May 22, 2018

A Different Approach to Energy-Harvesting Data Conversion

Energy harvesting for reporting sensor data takes many forms; this RF-powered temperature sensor exploits the terahertz band and frequency shift.

Energy harvesting is an important topic, as it opens up new options for data acquisition and monitoring. We now have transducers which can capture and convert the ambient energy of vibration, temperature, impact, and RF to electrical energy; ICs which can efficiently harvest and manage this energy; and processors and wireless links which operate at ultras-low power. Most of these applications also need a tiny battery for storing that captured energy and releasing it as operating power to the electronics. Depending on design and situation, the range of these harvesting installations can be fairly small — on the order of a few centimeters, but it can be more.

I’ve read about many harvesting designs and many are quite innovative and interesting. I came across one approach which seemed especially “out of the box” in "The world’s tiniest temperature sensor is powered by radio waves" from the University of Eindhoven, Netherlands (Technische Universiteit, Eindhoven, often referred to as TU/e). It was for a wireless temperature sensor which is powered entirely by impinging RF from the network with which it is associated.

The self-contained IC measures a tiny 2 × 2 mm and weighs under two milligrams, Figure 1. Present range is about 2.5 cm, but the researchers hope to extend it to 1 m within a year and ultimately to about 5 m — though that's an aggressive goal. It makes use RF as both power source and data interface.

Figure 1: This self-powered, THz-band temperature sensor has no battery or ultracapacitor, yet can convey temperature readings over a few centimeters (from the University of Eindhoven, the Netherlands).

While that's not a new idea, it does so with two interesting aspects: it operates in the terahertz band, and it communicates the sensed temperature value by varying the carrier frequency. The authors have posted a detailed PowerPoint presentation "Small Temperature Sensor Using mm-wave data and power transfer" which is quite informative and has block diagrams, schematics, photos, and graphs of performance.

The press release notes that the sensor has a specially developed router, with an antenna that sends radio waves to the sensors to power them. The sensor contains an antenna that captures the energy from the router. The sensor stores that energy and, once there is enough, the sensor switches on, measures the temperature and sends a signal to the router. This signal has a slightly distinctive frequency, depending on the temperature measured. The router can deduce the temperature from this distinctive frequency,Figure 2.

Figure 2: The change in the sensor's THz-band frequency output versus temperature makes use of carrier shift versus temperature, a parametric change which most engineers try to avoid.Source: University of Eindhoven

Using this temperature-to-frequency model is not itself a new idea, as V/F (voltage-to-frequency) A/D converters have been around for a long time, and there are even ICs which are designed to perform this specific A/D conversion. Usually, though, the V/F conversion is at baseband or at very low frequencies which greatly eases measuring the frequency using a simple timer, rather than using modulation of a terahertz carrier. It seems somewhat audacious to do it there, where measurement of even basic parameters is a challenge. It is also at odds with the usual desire for low temperature coefficient (tempco), since temperature drift of an operating point (in this case, a carrier frequency) is usually regarded as a problem which must be trimmed out, calibrated, or somehow compensated. Here, it is instead exploited and leveraged to provide a way to convey information.

Will this approach be adapted for widespread use? I certainly don’t know. But I do know that it is clever, intriguing, and shows that it's probably a good idea to get familiar with use of the terahertz band, as it offers both new avenues to solutions along with severe technical challenges. Sometimes called sub-mm waves, it lies between microwaves and infrared radiation and spans 0.3 to 3 THz (the 0.3 THz lower band edge is also called 300 GHz, using more-familiar terms). The terahertz band has a reputation of great potential for sensing situations but having had difficult real-world progress. (See the excellentIEEE Spectrumarticle "The Truth about Terahertz" which explains how some fundamental realities of physics give this band so much potential for innovation and yet induces so much exasperation and frustration in actual application.)

Do you see this type of self-powered sensor, using the THz band and temperature-sensing/frequency mapping as a viable approach? Is the THz band the next big area ready for major RF exploration and exploitation?

— Bill Schweber writes about all things engineering for EE Times and sister sites EDN and Planet Analog.